The fatigue load a tower of a wind turbine has to withstand is often a key parameter for the structural design of a wind turbine tower. If it is possible to reduce the fatigue load acting on the wind turbine tower then the tower could be manufactured with a less amount of steel which would result in a reduced cost and in a reduced weight of the tower. Alternatively, with a reduced fatigue load acting on the same wind turbine tower the tower could have a longer life time.
During the operation of a wind turbine there are two basically different directions of tower movements which result in fatigue loads acting on the tower of the wind turbine. A first fatigue load results from a tower top movement in a direction being horizontal and perpendicular to the direction of the wind. These tower top oscillations may be called side-side oscillations. A second fatigue load results from a tower top movement in a direction along the direction of the wind. These tower top oscillations may be called fore-aft oscillations.
EP 2146093 B1 discloses a method for damping oscillations of a tower of a wind turbine where the side-side tower fatigue load is reduced by damping the tower movement using a sinusoidal signal added to a power/torque reference signal used for controlling the operation of a wind turbine. The added signal will generate a sinusoidal component in the torque applied by the turbine generator which is located in the nacelle of the wind turbine. FIG. 5 shows that e.g. a clock-wise torque moves the wind turbine tower top to the right. By measuring the tower top movement, e.g. by using accelerometers, it is now straight-forward to calculate a signal which is effective for damping any existing side-side movement of the tower.
For actively damping in particular fore-aft oscillations of a tower of a wind turbine it is known to add a sinusoidal signal to a pitch angle reference signal used for operating the wind turbine. This causes a variation of the pitch angle of the rotor blades which leads to a change in the thrust force from the rotor affecting the tower top, such that it will have a sinusoidal force component. If the sinusoidal force has the correct phase it can be used to damp tower top fore-aft movements. This known procedure has the disadvantage that it results in an increased pitch activity (more movement of the pitch system) with typically a very high loading on the pitch system incl. blade pitch bearings of the pitch system of the respective wind turbine.
There may be a need for reducing in an easy and effective manner a fore-aft oscillatory movement of a tower of a wind turbine.